The present invention relates to a method and apparatus for calcining and more particularly to a method and apparatus for continuous calcining under a noxious gas atmosphere.
Fluorescent calcium sulfide (CaS) is an excellent material for forming the fluorescent screen of a cathode-ray tube because it is inexpensive and it has favorable characteristics when compared with other fluorescent materials. However, calcium sulfide also deteriorates when in contact with water during the application of the fluorescent screen, rendering it inadequate for practical use.
As a result, investigations have been performed concerning methods of treating fluorescent calcium sulfide to prevent its deterioration by water so that its excellent luminous efficiency can be utilized. Consequently, remarkable progress has been made concerning a number of red-colored calcium sulfide fluorescent materials. These materials are expected to be adopted for practical use in the near future.
Fluorescent calcium sulfide can be obtained by calcining calcium carbonate under a hydrogen sulfide atmosphere. However, it is quite difficult to continuously calcine the fluorescent calcium sulfide because hydrogen sulfide is extremely flammable and noxious, having a lethal dose of 1,000 to 1,500 ppm.
The previous method of calcining fluorescent calcium sulfide material is based on a batch process wherein a fixed quantity of calcium carbonate is calcined under an isolated hydrogen sulfide atmosphere. After the material is completely calcined, drawn out of furnace, and replaced by fresh calcium carbonate, this process is repeated. This type of batch processing, however, is not adequate for mass production.
In view of the prior art, there is a need for a method for continuous calcining under a noxious gas atmosphere as well as an apparatus to carry out such a method, enabling the mass production of fluorescent calcium sulfide.